In recent years, a digital coherent receiver has been developed to provide a large-capacity optical transmission system that enables long-haul transmission. A digital coherent receiver, which performs digital coherent reception, includes an optical front-end circuit and a digital signal processor. The optical front-end circuit generates a digital signal indicating a modulated optical signal using local light generated by a local light source. The digital signal includes a plurality of channels (I signal and Q signal respectively indicating an I-component and a Q-component of the modulated optical signal) for each polarization. The digital signal processor recovers data from the digital signal generated by the optical front-end circuit (i.e., I signal and Q signal). At this time, the digital signal processor may recover phase and/or compensate for dispersion.
In such a digital coherent receiver, skew (delay time difference) may be generated between digital signals of respective channels input to the digital signal processor. For example, the skew is caused by variation in length of signal lines between the optical front-end circuit and the digital signal processor or variation in characteristics of amplifiers provided for the respective channels. The skew may adversely affect the quality of signals recovered by the digital signal processor. The skew between the channels may also be generated in an optical transmitter.
Thus, a signal processing device for compensating for skew has been suggested. The suggested signal processing device includes a skew detector which detects a skew remaining between an in-phase signal and a quadrature signal output from a phase controller, and a control amount determining unit which determines a control amount in a phase controller by using a detection result of the skew by the skew detector and outputs the control amount to the phase controller (for example, Japanese Laid-Open Patent Publication No. 2010-193204).
To further improve spectral efficiency in an optical transmission system using a coherent receiver, a technique of increasing a multi-level degree of multi-level modulation or a technique of adopting OFDM has been studied. Also, to ensure the favorable quality of transmission by adopting such a technique, the quality of laser mounted in a transmitter and receiver has been improved. For example, laser frequency variation, phase noise, and line width have been improved.
By improving the quality of laser, however, characteristics variation in the transmission system may become obvious. For example, when skew is generated in both of an optical transmitter and an optical receiver and frequencies and phases of lasers mounted in the optical transmitter and the optical receiver substantially correspond to each other, the skew in the optical transmitter and the skew in the optical receiver may be accumulated or canceled by each other depending on operation conditions of the optical transmission system.
Thus, the optimum skew compensation amount is changed when the combination of the optical transmitter and the optical receiver is changed or when the system condition (laser frequency fluctuation or the like) is changed. Consequently, the characteristics of the optical transmission system may be deteriorated.